In the past, there have been developed multi-fuel internal combustion engines which are able to perform the mixed combustion of a plurality of kinds of fuels such as a liquid fuel (e.g., light oil, gasoline, etc.) and a gas fuel (e.g., compressed natural gas, hydrogen gas, etc.). For example, in a first patent document, there is disclosed an auxiliary fuel ignition type gas engine which uses natural gas as main fuel and light oil as auxiliary fuel. In this auxiliary fuel ignition type gas engine, mixed combustion of light oil and natural gas is carried out by using the light oil as an ignition source. In addition, in the first patent document, there is described a technique in which at the time of light load, only light oil is supplied to the internal combustion engine, and at the time of medium load and high load, light oil and natural gas are supplied to the internal combustion engine.
In a second patent document, there is disclosed a technique with respect to an internal combustion engine which is provided with a variable compression ratio mechanism which is able to change a mechanical compression ratio.